Re: [PATCH v1 1/1] x86/fred: Clear the WFE bit in missing-ENDBRANCH #CP

[Xin Li]

On 9/12/2024 12:57 AM, Peter Zijlstra wrote:
> On Wed, Sep 11, 2024 at 04:19:29PM -0700, Xin Li (Intel) wrote:
>
> > +static void ibt_clear_fred_wfe(struct pt_regs *regs)
> > +{
> > +	if (cpu_feature_enabled(X86_FEATURE_FRED))
> > +		regs->fred_cs.wfe = 0;
> > +}
> > +
> >   static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
> >   {
> >   	if ((error_code & CP_EC) != CP_ENDBR) {
> > @@ -90,6 +107,7 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
> >   
> >   	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_noendbr)) {
> >   		regs->ax = 0;
> > +		ibt_clear_fred_wfe(regs);
> >   		return;
> >   	}
> >   
> > @@ -97,6 +115,7 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
> >   	if (!ibt_fatal) {
> >   		printk(KERN_DEFAULT CUT_HERE);
> >   		__warn(__FILE__, __LINE__, (void *)regs->ip, TAINT_WARN, regs, NULL);
> > +		ibt_clear_fred_wfe(regs);
> >   		return;
> >   	}
> >   	BUG();
>
> So, why not clear the bit the moment we know this is CP_ENDBR?
>
> In the fatal case, we'll hit that BUG and die anyway, nobody cares about
> the tracker state in that case.

I did think about it, but preferred to leave it as in BUG().

But as you have asked, will change unless someone else disagrees with a
good reason :)

> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> index d2c732a34e5d..fde4bdd25a73 100644
> --- a/arch/x86/kernel/cet.c
> +++ b/arch/x86/kernel/cet.c
> @@ -88,6 +88,8 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
>   		return;
>   	}
>   
> +	ibt_clear_fred_wfe(regs);
> +
>   	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_noendbr)) {
>   		regs->ax = 0;
>   		return;